chore: extend issue stale window to 28 days #4961

User Story

As an OpenShell repository maintainer, I find that issues are becoming stale too quickly and need a 28-day inactivity window while retaining the existing 14-day pull request window.

Description

Extend the time before an issue is marked state:stale from 14 to 28 days. Keep the subsequent 14-day issue closure window and all pull request stale behavior unchanged. Exempt issues labeled roadmap from stale marking and automatic closure alongside the existing frozen-label exemptions. Update the automated issue message and contributor guidance to match.

Context

The stale workflow currently marks both issues and pull requests stale after 14 days. Issues then close after another 14 days; pull requests do not close automatically.

Definition of Done

• Issues become stale after 28 days without activity.
• Issues still close after another 14 days without activity.
• Pull requests still become stale after 14 days and do not close automatically.
• Issues labeled roadmap are exempt from stale handling.
• Automated messages and contributor documentation reflect the policy.